A gas membrane separation process is attracting attention in recent years because it can realize energy saving as a process for separating an acidic gas from synthetic gases synthesized in large-scale plants such as hydrogen production, urea production and the like, natural gas, exhaust gas and the like. Various acidic gas separation membranes for the gas membrane separation process are being studied. For example, patent document 1 describes an acidic gas separation membrane containing an amine carrier.